One of the problems encountered, in the field of signalling and lighting a road by a vehicle, concerns the visibility of the road in fog. It is known in fact that in foggy weather the visibility of the road scene situated at the front of the vehicle is reduced, which can cause accidents. In order to improve the visibility of the road scene, in foggy weather, current vehicles are equipped with fog light devices at the front of the vehicle and fog lights at the rear of the vehicle.
Switching lighting devices and fog lights on and off is generally controlled manually by the driver from switches disposed on the vehicle dashboard. However, it may happen that the driver forgets to switch on his lights and fog lights. Failure to switch on the fog lights may prove to be dangerous for the vehicle situated behind the vehicle in question. The lack of switching on of the fog lighting devices causes poor visibility of the road scene situated in front of the vehicle with all the risks that this entails.
Conversely, the improper use of fog lights, for example in rain, may cause a nuisance to the driver situated behind the vehicle in question. Likewise the improper use of fog lights at the front of the vehicle may be a nuisance for the drivers coming in the opposite direction in the absence of fog. On the other hand, in the case of intense fog, it may be advantageous for the driver of the vehicle to be able to have available sufficiently powerful lighting to afford correct visibility of the road scene.
There currently exist devices for detecting the presence of fog for automatically controlling the switching on of the lights and illumination devices and enabling the driver to adapt the speed of his vehicle according to the visibility that he has of the road scene situated in front of the vehicle. One of these devices for detecting the presence of fog uses an anti-collision system of the LIDAR (Light Detection and Ranging) type, which makes it possible to evaluate the transmission of the atmosphere in order to deduce therefrom the presence of fog. However, LIDARs are very expensive systems; installing them is therefore difficult to envisage on conventional vehicles.
There exists, moreover, a method for detecting the presence of fog and measuring the visibility distance in daytime conditions. This method is described in the article entitled “Driving assistance: automatic fog detection and measure of the visibility distance” by N Hautiere and D. Aubert. It is based on Koschmieder's law, which provides a simple expression of the luminance of an object observed at a distance d:L=L0exp(−kd)+Lf(1−exp(−kd))where L0 is the intrinsic luminance of the object, k is the coefficient of extinction of the fog and Lf is the luminance of the ambient fog caused by the many diffusions of the light in the atmosphere. The parameters of the Koschmieder model must be calculated on homogeneous sky/road regions. However, this method is difficult to use for the detection of nocturnal fog, since the parameters used for extracting the homogeneous regions by day are much less marked at night. Locating these regions is therefore difficult. In addition, in night-time conditions, the illumination of the road scene is achieved by the vehicle headlights and not by light coming from the sky. There then appears a light halo around the headlights, which makes it difficult to us Koschmieder's law.
There also exist a method and system described in patent application FR-A-2 847 367. This method is based on the search for a homogeneous region in an image of the road scene and on the search for a vertical light gradient. This search makes it possible to establish a curve, at the reversal point of which a relationship is obtained between the visibility distance and the reversal point. More precisely, this method comprises a step of separating the image into two parts by a vertical line passing substantially through the middle of the image, a step of determining the luminosity of the pixels of the vertical line according to the position of the pixels on the vertical line, a step of calculating the reversal point of the curve representing the luminosity of the pixels according to their position; and a step of determining the visibility distance of the driver from the visibility distance of the driver from the position of this reversal point on the image.
In this method, it is considered that the light comes from the sky and that the dark area is situated close to the ground. The separation between the dark area and the light area is then made by a line. This method is therefore adapted solely for the detection of daytime fog and not for the detection of night-time fog. This is because, in nocturnal conditions, the light does not come from the sky; it comes from the vehicle headlights. However, the light emitted by the headlights forms, in the presence of fog, light halos in a non-planar form. The light emitted by the headlights of a vehicle cannot therefore be separated by a horizontal straight line.
In addition, at night, it is important to take into account the fact that more and more lighting devices comprise a DBL (Dynamic Bending Light) system which provides pivoting of the headlight according to the path of the road. For example, with a DBL system, the headlights pivot to the right of the axis of the vehicle when there is a bend to the right and to the left when there is a bend to the left. The area illuminated by the vehicle headlights is therefore not constant, that is to say not necessarily in the axis of the vehicle.